Look closely in certain supermarket aisles and you might be baffled by what's touted on the packaging. "Free of 1,4-dioxane," reads the label on one eco-friendly toilet-bowl cleaner. A leading hair-care brand offers "sulfate free" shampoo. It's only after you breathe a sigh of relief that ingredient anxiety kicks in: Wait, what's wrong with sulfate? And how many products is it in?

As grocery-store shoppers become more label conscious, many companies are highlighting--often in big type--what their products don't contain. Some of these noningredients, like BPA, have made headlines in recent years, while others are so obscure that few consumers know...